A.) Field of Use
The present invention relates to catalysts, systems, and methods that are useful for treating an exhaust gas which occurs as a result of combustion of hydrocarbon fuel, and particularly exhaust gas containing nitrogen oxides, such as an exhaust gas produced by diesel engines.
B.) Description of Related Art
The largest portions of most combustion exhaust gases contain relatively benign nitrogen (N2), water vapor (H2O), and carbon dioxide (CO2); but the exhaust gas also contains in relatively small part noxious and/or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC) from un-burnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, and particulate matter (mostly soot). To mitigate the environmental impact of exhaust gas released into the atmosphere, it is desirable to eliminate or reduce the amount of these undesirable components, preferably by a process that, in turn, does not generate other noxious or toxic substances.
One of the most burdensome components to remove from a vehicular exhaust gas is NOx, which includes nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). The reduction of NOx to N2 in a lean burn exhaust gas, such as that created by diesel engines, is particularly problematic because the exhaust gas contains enough oxygen to favor oxidative reactions instead of reduction. NOx can be reduced in a diesel exhaust gas, however, by a process commonly known as Selective Catalytic Reduction (SCR). An SCR process involves the conversion of NOx, in the presence of a catalyst and with the aid of a reducing agent, into elemental nitrogen (N2) and water. In an SCR process, a gaseous reductant such as ammonia is added to an exhaust gas stream prior to contacting the exhaust gas with the SCR catalyst. The reductant is absorbed onto the catalyst and the NOx reduction reaction takes place as the gases pass through or over the catalyzed substrate. The chemical equation for stoichiometric SCR reactions using ammonia is:2NO+4NH3+2O2→3N2+6H2O2NO2+4NH3+O2→3N2+6H2ONO+NO2+2NH3→2N2+3H2O
Known SCR catalysts include zeolites and other molecular sieves. Molecular sieves are microporous crystalline solids with well-defined structures and generally contain silicon, aluminum and oxygen in their framework and can also contain cations within their pores. A defining feature of a molecular sieve is its crystalline or pseudo-crystalline structure which is formed by molecular tetrahedral cells interconnected in a regular and/or repeating manner to form a framework. Unique molecular sieve frameworks recognized by the International Zeolite Association (IZA) Structure Commission are assigned a three-letter code to designate the framework type. Examples of molecular sieve frameworks that are known SCR catalysts include Framework Type Codes CHA (chabazite), BEA (beta), and MOR (mordenite).
Some molecular sieves have a three-dimensional molecular framework that arises from the orientation of their interconnected cells. The cells of these molecular sieves typically have volumes on the order of a few cubic nanometers and cell openings (also referred to as “pores” or “apertures”) on the order of a few angstroms in diameter. The cells can be defined by the ring size of their pores, where, for example, the term “8-ring” refers to a closed loop that is built from 8 tetrahedrally coordinated silicon (or aluminum) atoms and 8 oxygen atoms. In certain zeolites, the cell pores are aligned within the framework to create one or more channels which extend through the framework, thus creating a mechanism to restrict the ingress or passage of different molecular or ionic species through the molecular sieve, based on the relative sizes of the channels and molecular or ionic species. The size and shape of molecular sieves affect their catalytic activity in part because they exert a steric influence on the reactants, controlling the access of reactants and products. For example, small molecules, such as NOx, can typically pass into and out of the cells and/or can diffuse through the channels of a small-pore molecular sieve (i.e., those having framework with a maximum ring size of eight tetrahedral atoms), whereas larger molecules, such as long chain hydrocarbons, cannot. Moreover, partial or total dehydration of a molecular sieve can results in a crystal structure interlaced with channels of molecular dimensions.
Molecular sieves having a small pore framework, i.e., containing a maximum ring size of 8, have been found to be particularly useful in SCR applications. Small pore molecular sieves include those having the following crystalline structure types: CHA, LEV, ERI, and AEI. Specific aluminosilicates and silico-aluminophosphates examples of molecular sieves having the CHA framework include SAPO-34, AIPO-34, and SSZ-13.
Zeolites are aluminosilicate molecular sieves having a crystalline framework of interconnected alumina and silica, in particular, cross-linked alumina and silica via a sharing of oxygen atoms, and thus can be characterized by its silica-to-alumina ratio (SAR). In general, as a zeolite's SAR increases, the zeolite becomes more hydrothermal stability. Since the temperature of an exhaust gas exiting a mobile lean-burn engine, such as a diesel engine, is often 500 to 650° C. or higher and typically contains water vapor, hydrothermal stability is an important consideration in designing an SCR catalyst.
While zeolites per se often have catalytic properties, their SCR catalytic performance may be improved in certain environments by a cationic exchange wherein a portion of ionic species existing on the surface or within the framework is replaced by metal cations, such Cu2+. That is, a zeolite's SCR performance can be promoted by loosely holding one or more metal ions to the molecular sieve's framework.
It is also desirable for an SCR catalyst to have high catalytic activity at low operating temperatures. At low operating temperatures, for example below 400° C., a higher metal loading on a molecular sieve results in higher SCR activity. However, the achievable metal loading is often dependent on the quantity of exchange sites in the molecular sieve, which in turn is dependent upon the material's SAR. In general, molecular sieves with low SAR allow for the highest metal loadings, thus leading to a conflict between the need for high catalytic activity and high hydrothermal stability which is achieved by a relatively higher SAR value. Moreover, high copper-loaded catalysts do not perform as well at high temperatures (e.g., >450° C.). For example, loading an aluminosilicate having a CHA framework with large amounts of copper (i.e., copper-to-aluminum atomic ratio of >0.25) can result in significant NH3 oxidation at temperatures over 450° C., resulting in low selectivity to N2. This shortcoming is particularly acute under filter regeneration conditions which involves exposing the catalyst to temperatures above 650° C.
Another important consideration in designing an SCR catalyst for mobile application is the performance consistency of the catalyst. For example, it is desirable for a fresh catalyst to produce a similar level of NOx conversion to the same catalyst after it has aged.
Accordingly, there remains a need for SCR catalysts that offer improved performance over existing SCR materials.